Still conventionally today, in many car models, the upper seat belt attachment point is located on the structure of the car, generally on a door post for the front seats. As the longitudinal position of these seats is classically adjustable, that is in the front-rear direction, the position of the belt is not always suitable to the morphology of the user and selected seat position because the relative positioning of the belt anchoring point in relation to the seat varies according to the longitudinal position of the seat in the car.
This is why so-called "incorporated belt" seats are being developed which incorporate the seat belt into their structures so that the relative position in relation to the seat is independent of the position of the seat in the car. Also note that this principle can be advantageously used for the removable seats used in people-carrier type vehicles, ie monospace cars.
The 3-point type incorporated seat belt is conventionally attached to the seat by two lower attachment points, located on either side of the seat, towards the rear of the seat pan, and an attachment point located at the top of the backrest, on one side. Normally, this attachment point is in fact used to change the direction of the belt the end of which is connected to a self-winding reel housed either in the backrest itself or in the seat pan. For harness-type seat belts, there are two upper attachment points located at the top and on either side of the backrest. In seats with incorporated belts, a major concern is to ensure the resistance of the seat backrest to the high loads which may result from the tensile forces exerted by the belt on the top of the backrest during a shock or a strong deceleration of the vehicle. For this, at least one of the lateral uprights of the backrest framework, that is the one supporting the upper attachment point of the belt, consists of a beam or post the mechanical strength of which is designed to take the bending loads generated by the tensile forces exerted on the belt. If the vehicle is involved in an accident, these loads can be very high. To enable the use of a seat with two upper attachment points, or simply to ensure seat symmetry, enabling manufacturing costs to be limited, the two lateral uprights of the backrest can consist of such a beam with high mechanical strength.
The backrest framework is completed by structural elements, such as cross members or various supports, in particular elements supporting a headrest, connected to the two uprights of the backrest and intended to ensure the rigidity and lateral stability of the backrest as a whole.
Such a structure does not offer the possibility of adjusting the position of the upper section of the backrest, that is, the section located more or less level with and above the shoulders of the user. Indeed, such an adjustment, which requires a hinge between the lower section of the backrest framework and its upper section, would require the incorporation of such a hinge in the beam or beams forming the uprights of the framework. It can be easily understood that the use of such a hinge would be detrimental to the mechanical strength of the framework and therefore to the safety provided by the belt as the upper attachment point of the belt would then necessarily be transferred to the hinged section so as to locate it, as required, more or less just above the shoulder of the user. In other words, such an arrangement would require that all the loads exerted by the belt on the top of the seat be passed via the said hinge and via the adjustment and locking mechanisms necessarily associated with it.
Also, to be able to supply, if applicable, seats of the same model but some equipped with the backrest top section adjustment and others not, it would be necessary, for manufacture, to use two sets of different tools, each set adapted to one of the two framework versions.